Jonathan Mitchell is an epidemiologist whose research aims to explain variations in childhood growth patterns. His long-term career goal is to lead a program of biobehavioral research to test how behaviors and genes combine to affect body composition and the development of obesity during adolescence. His work will help to inform childhood obesity prevention and promote healthy bone development. This career development award will provide him with training in sleep and he will develop expert knowledge on the integrated relationships between sleep, dietary intakes and physical activity in childhood. He will also receive training on the genetics of childhood obesity and bone mass, and this will enable him to lead gene-environment interaction studies. Finally, he will receive expert training on the methodologies used to measure body composition in the lab setting so that he can specifically estimate adiposity and bone mass in his research studies. His development in these areas will be achieved through a series of training activities and a research project, guided by an interdisciplinary team of senior scientists with expertise in sleep, genetics, nutrition, physicl activity, and body composition. He will complete coursework and attend seminars at the University of Pennsylvania that will cover sleep, nutrition, and obesity research. He will collaborate with members of his mentoring team to complete laboratory rotations and independent studies on sleep measurement, body composition measurement, and the genetics of body composition. He will complete short courses that cover physical activity research and longitudinal data analysis, and he will attend annual scientific meetings hosted by the American Academy of Sleep Medicine (SLEEP), the American Heart Association (Epidemiology and Prevention/Lifestyle and Cardiometabolic Health [EPI/Lifestyle]), and the Obesity Society (Obesity Week). He will lead a research project that will use resources at the University of Pennsylvania, Perelman School of Medicine and the Children's Hospital of Philadelphia (CHOP). The research project aims to investigate the association between sleep, adolescent obesity and energy intake and physical activity. He will recruit a sample of 100 healthy adolescents aged 13 to 14 (8th grade) at baseline and will re-assess these adolescents at 1-year follow-up (9th grade. The participants have already been genotyped for single nucleotide polymorphisms (SNPs), representing obesity susceptibility loci, at the Center for Applied Genomics at CHOP. At both time points, he will provide each participant with two accelerometers; one will be worn for 24 hours per day for 2-weeks on the wrist to estimate total sleep time (TST). The second will be worn on the hip during waking hours for 2-weeks to estimate time spent in moderate-to-vigorous physical activity (MVPA) and sedentary behavior (SB). During the 2-week wear periods trained Nutrition Core staff will call each participant on three occasions to record foods consumed in the past 24 hours; these data will be used to estimate energy intakes. Each participant will visit the CTRC Nutrition Core to have his or her fat mass estimated by dual energy X-ray absorptiometry (DXA). He hypothesizes that: 1) declines in TST will be associated with higher fat mass, 2) declines in TST will be associated with increased energy intake, and 3) declines in TST will be associated with less time spent in MVPA and more time spent in SB. This longitudinal study will provide the experience and necessary pilot and feasibility data to support a larger R01 study of the effects of sleep, diet and physical activity on adiposity changes during adolescence.